As the number of subscribers increase together with the performance requirements in communication networks, so does the requirement on coordination in the communication network. If, for example, certain parts of the communication network (e.g., certain cells or cell sectors) are crowded with lots of mobile terminals (UEs) while other parts (e.g., other cells) are not, the network capacity will be left unutilized since the crowded cells, which have only a portion of the total network capacity, will limit its users and UEs, respectively, while at the same time other cells or cell sectors, that are (close to) empty, are under-utilized.
An example of such a communication network is the architecture of the LTE system as shown in FIG. 1, including radio access nodes (eNBs), also referred to as a network node in the following, and evolved packet core nodes (MME/S-GW) as well as logical interfaces between eNBs (X2) and between eNBs and MME/S-GWs (S1). The present description uses the LTE system as an example, but generalizes to other wireless networking systems and standards as well. In more general terms, such a network node may provide a radio access to a mobile terminal (UE) and also has some kind of coverage area in which it may provide the radio access. Such a radio access may be of any spectrum or standard (GSM, GPRS, 3G, 4G, LTE, WiFi, even DECT, etc.). The mobile terminal (UE) may thus be connected to one or more radio access nodes in the communication network, and may be supported by one or more core network nodes. Furthermore, the UE may be served via one or more frequency carriers, and one or more radio access technologies.
One feature aimed to solve the problem of coordination in the communication network is load balancing where different cells, that overlap (in communication terms, but could also overlap in geographical terms), share the current load on the network. This feature moves UEs from the high-loaded cells to the less loaded cells. More specifically, in order to increase a capacity in the communication network the operator may deploy cells on multiple frequency layers, also referred to as carriers, as illustrated in FIG. 2. Load balancing is a technique to balance the traffic load between overlaid cells in the communication network in order to utilize the capacity on the different frequency layers. In FIG. 2, potential load balancing opportunities are indicated by respective arrows and include load balancing opportunities between co-located cells as served by the same eNB and between non-co-located cells as served by different eNBs. In general, each eNB assesses the traffic load in its cells. The traffic load information is exchanged between the cells and/or eNBs, after which a load balancing algorithm identifies whether there is a need to move UEs between the cells in order to balance the traffic load. If there is a need to move UEs in order to balance the traffic load, UEs are selected and ordered to perform inter-frequency measurements in order to be moved in some way (for example, handover, release with redirect, etc.).
Another feature for solving this coordination problem is carrier aggregation (CA) where different carriers are aggregated for one UE. This enables a UE to transmit/receive data over more than one frequency layer/carriers, and thus provides a higher capacity to the UEs. When multiple carriers are available, it is possible to deploy several cells with similar coverage area as illustrated in FIG. 3 which shows the case of co-located cells being served by the same radio base station (eNB), or with different coverage area, each at a different carrier than the other (as illustrated by FIG. 2). Each carrier is referred to as a Component Carrier (CC).
In CA, two or more such CCs are aggregated in order to support wider transmission bandwidths. A UE may simultaneously receive or transmit on one or multiple CCs depending on its capabilities. A UE with reception and/or transmission capabilities for CA can simultaneously receive and/or transmit on multiple CCs corresponding to multiple serving cells. Here, CA is specified for both contiguous and non-contiguous CCs.
It is possible to configure a UE to aggregate a different number of CCs originating from the same eNB and of possibly different bandwidths in the UL and the DL. The number of DL CCs that can be configured depends on the DL aggregation capability of the UE and the network. The number of UL CCs that can be configured depends on the UL aggregation capability of the UE and the network. CCs originating from the same eNB need not to provide the same coverage. An example is shown in FIG. 4 illustrating a radio base station (eNB) with several co-located cells configured as different CC for a UE, where one CC is the Primary cell (PCell), and two further CCs are Secondary cells (SCells).
A UE may indicate to the serving radio base station (eNB) its capability to support one or more SCells in the DL, as well as one or more SCells in the UL via RRC signaling (see Chapter 18 in 3GPP TS 36.300, V13.2.0). Further, CA capable mobile terminals/devices (UEs) may be configured with one or more SCells in UL and/or DL via RRC signaling, using the message RRCConnectionReconfiguration and the information element sCellToAddModList-r10 in 3GPP TS36.331 V13.0.0.
Moreover, CA configured mobile devices (UEs) may have one or more of its SCells activated via signaling from the serving radio base station, using an activation MAC control element. CA activated mobile devices may have data scheduled at one or more of its SCells via control signaling from the radio base station. This control signaling may be sent via the physical control channel and MAC control element, and may refer to a DL SCell resource and/or an UL SCell resource. Moreover, if cross-carrier scheduling is supported, the SCell resource assignment is signaled to the mobile device (UE) via the PCell physical control channel and/or MAC control elements. When the UE is scheduled over two or more CCs, the data will be mapped to PRBs both on the PCell as well as the SCell(s).
A typical way of finding the right UEs for inter-frequency handover or carrier aggregation is to let the network request a number of UEs to measure the radio conditions (such as RSRP) in the target carrier, and to find the potential target cells for the chosen UEs. After the UEs have conducted their measurements, the reports are fed back to the serving cell which then takes a decision which of the UEs to move to which cell.
Further, the concept of Dual Connectivity (DC) as introduced in 3GPP in Rel. 12 enables the establishment of user plane connections via another radio node, referred to as a Secondary eNB (SeNB), while maintaining the higher layer connection management (RRC) via a Master eNB (MeNB). This means that a device (UE) may have user plane connections completely via the MeNB, completely via the SeNB, or are split between both MeNB and SeNB, as illustrated in FIG. 5a. 
Furthermore, it is also possible to aggregate several component carriers at the MeNB (the CCs are referred to as a MeNB Cell Group (MCG)) as well as at SeNB (the CCs are referred to as a SeNB Cell Group (SCG)). Each cell group comprises one or more CC. At the MeNB, these are denoted MCG PCell and MCG SCell, while at the SeNB, these are denoted SCG PSCell and SCell. Note that the PSCell via SeNB has some of the scope of a MCG PCell, but not all. For example RRC signaling is only handled via the PCell. The cell groups are illustrated by FIG. 5b. 
The cell group notation may be also used when labelling data radio bearers (DRB)—a DRB via MeNB is referred to as a MCG DRB, a DRB via SeNB is referred to as a SCG DRB and a DRB split between both MeNB and SeNB is referred to as a Split DRB. When generalizing the concept of dual connectivity, it may also be possible to consider connections to more than one SeNB, but also where the SeNB supports a different radio access technology, such as WiFi, 5G, 3G, 2G, etc.
Furthermore, an unlicensed carrier operation may be different from licensed carrier operation in that there are some co-existence criteria that have to be met in order to co-exist with other connections in the same band. This is discussed in 3GPP (TR 36.889 V 13.0.0) as Licensed Assisted Access (LAA), which essentially is a PCell operating on a licensed carrier and an SCell operating on an unlicensed carrier, where the SCell then also has to meet the co-existence criteria. FIG. 6 illustrates some different deployment scenarios in that context.